The invention relates to a method for taking a spatially resolved spectrum, in particular infrared (IR) spectrum, of a sample by means of a Fourier-transform (FT)-spectrometer, wherein light emitted by a light source is fed to an interferometer, directed onto the sample and detected by an array-detector, wherein a movable reflector of the interferometer is displaced along a distance s and the array-detector is read out at a number of predetermined discrete way points s1, . . . , sn of the distance s.
The invention further relates to a FT-spectrometer, in particular for carrying out the method.
Such a method and a FT-spectrometer are known from their general use.
In FT-spectroscopy, whether spatially resolved or not spatially resolved, the light emitted by the light source is splitted by a semi-permeable beam splitter of the interferometer into two beam parts, which are reflected at a fixed and a movable reflector and brought to interference after recombination. By displacing the movable reflector, the optical path length in one of the interferometer arms is changed which results in a phase difference between both beam parts and, thus, in a change of the interference amplitude. The curve of the intensity of the light as a function of the optical path difference, which is also referred to as interferogram, is detected by the detector. Thus, the distribution of the intensity of the light coming from the sample is first measured in the space domain and subsequently converted into the real spectrum by means of a subsequent Fourier transformation which is carried out by a computer.
The interferogram mentioned before is stored point-by-point rather than being stored continuously. For this purpose, it is usual to simultaneously detect the interference pattern of a monochromatic laser, e.g. a helium-neon laser, the radiation of which is also led in the optical working beam path, for example, by an additional detector diode. The zero passages of the laser sine signal or a multiple of these intervals define the way points, at which the interferogram is stored in digitized manner. For this purpose, the movable reflector of the interferometer is displaced along a predetermined distance, and the respective measuring signal of the sample is read out from the detector at the predetermined discrete way points of the movable reflector.
For taking a spatially resolved spectrum, an array-detector is used, from which all the pixels or voxels have to be read out at each way point, which involves a considerable volume of data entailing the problems described hereinafter when the array-detector is read out at the way points.
For displacing the movable reflector of the interferometer two fundamental technics are known.
In the so-called step-scan-method, the movable reflector is displaced along the distance in discrete steps to the single way points, i.e. the movable reflector is stopped at each of the single predetermined discrete way points, and the array-detector is then read out during the standstill of the reflector at the way points. The disadvantage of this method is that the movable reflector has to be repositioned each time, which entails a downtime during which no data can be picked up by the array-detector. What is more, the exact position of the reflector has to be first waited for, before the data acquisition can begin. The downtimes caused by the positioning of the reflector are in the range of several milliseconds per reflector position, which accordingly sum up according to the usually high number of way points.
In the so-called rapid-scan-method, the movable reflector is continuously displaced, and the array-detector is read out during the displacement of the reflector. However, it has to be taken into consideration that the available array-detectors, for example focal-plane-array-detectors, allow for a data transfer rate in the order of 200 Hz only. This low data rate of available array-detectors requires a very slow rate of displacement of the interferometer reflector in this continuous scan method, which is at 0,1 mm/s (or less) which entails a very sensitive motion of the reflector and, thus, an unprecise pick-up of the spectrum. A rate or speed of displacement of the reflector of about 5 cm/s, as it is usually used by FT-spectrometers for non-spatially resolved spectroscopy, is desirable. However, the low data rate of the array-detectors does not allow for such a high rate of displacement.
It is common to both pick-up technics mentioned before that the measuring signals are picked up sequentially, i.e. all single way points are approached one after another when the movable reflector is displaced along the predetermined distance, and, when doing so, the required image data are picked up sequentially.
It is, therefore, an object of the invention to improve a method of the kind mentioned at the outset such that the disadvantages of the prior art are avoided, in particular that a precise pick-up of the spatially resolved spectrum is rendered possible.